In facsimile apparatus, document reproduction is generally performed by a print head that scans an image supporting medium in successive scan lines. The medium is often some kind of sensitive paper. The print head is excited by data representative of points detected along line of a document to be reproduced. An image of the document is produced on the medium by causing corresponding points to appear thereon by means of whatever printing method is employed by the apparatus.
Some printing methods require a sensitive medium whose appearance can be changed locally, e.g. by changing its color. Such media include electrochemical, electrothermal and heat sensitive papers. An image of the document to be reproduced is then formed by changing the appearance of those points on the sensitive medium which are determined by the data representative of image points on the original document. Thus, when using an electrochemical or an electrothermal paper, local appearance is modified by applying an electrically conductive stylus to the paper and passing a current between the stylus and the paper. The passage of electricity through the paper changes its color, either by inducing a chemical reaction or by resistive heating, depending on whether the paper is electrochemical or electrothermal. When using heat sensitive paper, the stylus includes a resistive element through which the electric current is passed, thereby heating the stylus. A local change of color is produced on the heat sensitive paper by a physical or chemical reaction which is induced by the hot stylus being pressed thereagainst, or being brought very close thereto.
Another printing method is the electrostatic method which makes use of an image supporting medium comprising an electrically conductive substrate covered with a thin dielectric layer. In this method, a latent image of the document to be reproduced is formed on the medium by placing electric charges at points on the medium that are defined by the data representing the document to be reproduced. Localised charges can be placed on the medium by applying both an electrode and a counter-electrode against the medium, and then setting up a potential difference of several hundreds of volts between the electrode and the counter electrode. The electrode is constituted by the stylus and it is pointed, while the counter electrode is larger and flat. The electrode and counter-electrode may both be disposed on the same side of the medium, in which case they are disposed on the dielectric side, or else they may be on opposite sides, in which case the electrode is on the dielectric side and the counter-electrode is on the conductive side. The potential difference ionizes air in the neighborhood of the pointed electrode, and since the counter-electrode is larger and flat, electric charge accumulates locally on the dielectric layer in the vicinity of the electrode. The latent image thus formed is then developed and fixed, either on the original medium (e.g. a dielectric paper) or on another medium (e.g. ordinary paper) after transfer thereto. Diverse techniques are used for doing this, but they are not described further here since they have no direct bearing on the invention.
Some print heads have as many styluses as there are image points along a scan line. The styluses may be aligned in a single line, in which case they are spaced at the point definition pitch along the scan line, or else they may be disposed in two parallel lines, in which case they are spaced at twice the point deflection pitch along the scan line, with the styluses of one line being placed opposite the gaps in the other. The lines of styluses may be separated by an isolation line whose width is equal to the width of one scan line. Regardless of the number of lines of styluses, an image is printed by relative movement of the medium and the print head in a direction perpendicular to the lines of styluses.
Since the image definition path along a scan line is generally very small (125 .mu.m for group III facsimile apparatus), industrial production of such prior multi-stylus print heads calls for highly specialised tooling.
Preferred embodiments of the present invention provide a multi-stylus print head that can be manufactured using more conventional tooling.